When this happens, an opportunity presents itself.
"Many exoplanets have been discovered orbiting other stars, but by watching the transit here, we can compare this with exoplanets passing in front of other stars," said Miguel Pérez Ayucar, of ESA's European Space Astronomy Centre (ESAC) in Spain, while observing the event near the summit of Paranal.
ANALYSIS: Mercury Transit: Smallest Planet to Make BIG Entrance
We can use the Mercury transit as an exoplanet analog, to see how the transit looks from Earth and then comparing that with transits of distant worlds orbitng other stars.
Missions such as NASA's Kepler space telescope watch for the slight dimming of the light of stars, revelaing that an exoplanet has paassed in front, blocking a tiny amount of starlight. This is exactly what Mercury did on Monday (and Venus last did in 2012), so these transits close to home can be used as a tool to understand transits far away.
Also, the Mercury transit can be used to study the planet's exosphere - an extremely thin atmosphere that fizzes into space. Sunlight that passes through the exosphere is encoded with spectroscopic data that can then be used to work out the nature of the environment surrounding Mercury. As we've had missions that have explored the planet, such as NASA's MESSENGER mission (and future ESA BepiColumbo) we have the "ground truth" as to the characteristics of that exosphere, so we can compare direct measurements with transit observations to better calibrate astronomical techniques.